This invention relates generally to analog to digital converters (ADC) and, more particularly, the invention relates to reference voltage control in an ADC.
As illustrated in FIG. 1, an ADC 10 converts an analog signal 12 to digital form at 14 by comparing the analog data to reference voltages to drive a code converter and mapping it to a representative code. The reference voltages are established by a gain digital-analog converter (DAC) 16 and an offset DAC 18. When the data comes from different measurements in a physical system, the reference voltages must be precalibrated for each datum source so that the reference voltage span correlates to the span of the datum source and a zero offset correlates to a zero value of the datum source. For example, in a charge coupled device (CCD) each pixel of an image requires a full scale and a black level correction to compensate for differences in gain and zero (black level) of CCD outputs. By precalculating the detectors in the physical system, digital codes can be used to set a zero reference voltage level and a voltage span for individual measurements.
Heretofore, conventional voltage DACs using fixed voltage references have been employed to obtain the full scale and zero reference voltages, as shown schematically in FIG. 2. The precalculated (or calibrated) digital codes of the control data drive the span (gain) DAC 20 and the zero (scale) DAC 22 which drive voltage followers 24, 26 to establish the reference voltage across the ladder resistor 28. The operational amplifiers of the voltage followers are costly in integrated circuit surface area, increase power consumption, and limit the achievable settling time of the reference. Further, independent control of the zero and full scale reference is desired, but the traditional voltage mode implementation results in interaction between the references whereby a change in the zero reference point also affects the total span. Further, the operational amplifier sees a load impedance in the range of hundreds or a few thousand ohms which results in a wide range of output current which leads to large power supply transients, as well as requiring an idle current to keep the circuit ready for higher speed operation. Further, the operational amplifier can require both plus and minus voltage supplies.